Fair Value Measurements (Schedule Of Level 3 Financial Instruments) (Details) - Fuel swap contracts $ in Millions		Mar. 31, 2020 USD ($) $ / gal	Dec. 31, 2019 USD ($) $ / gal	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Information relating to the significant unobservable inputs of Level 3 financial instruments
Fair value | $		$ (8)	$ 1		$ (4)
Discounted Cash Flows | Minimum [Member]
Information relating to the significant unobservable inputs of Level 3 financial instruments
Forward Unleaded Price per Gallon (in dollars per gallon)	[1]	1.35	2.37
Discounted Cash Flows | Maximum [Member]
Information relating to the significant unobservable inputs of Level 3 financial instruments
Forward Unleaded Price per Gallon (in dollars per gallon)	[1]	1.96	2.80
Discounted Cash Flows | Weighted Average
Information relating to the significant unobservable inputs of Level 3 financial instruments
Forward Unleaded Price per Gallon (in dollars per gallon)		1.75	2.61

[1] Forward prices per gallon were derived from third-party market data providers. A decrease in the forward price would result in a decrease in the fair value of the fuel swap contracts.